The present invention relates generally to optoelectronic transceivers, and particularly to optical assemblies within the optoelectronic transceivers that allow the optical fibers to be spaced very close to each other.
One of the emerging standards in data communications and telecommunications uses a family of connectors called MT-RJ which have a pair of very closely spaced optical fibers. In particular, these connectors have a 0.75 mm (which may also be expressed as 750 xcexcm) fiber pitch, which means that the centers of the two fibers in these connectors are 0.75 mm apart, within a tolerance of about xc2x15 xcexcm for multimode applications and about xc2x11 xcexcm for single mode applications.
The transistor outline (TO) packages (sometimes called TO cans) in which most optical sources (e.g., laser diodes) and optical detectors (i.e., photodiodes) are housed have a diameter of at least 5 mm, and these TO packages are furthermore often embedded in mechanical port assemblies that provide for the mechanical alignment of the TO packages. These mechanical port assemblies are usually substantially larger than the TO packages. Each optical element (e.g., laser diode or photodiode) in its TO package housing (which may or may not include a port assembly as well) is herein called an optical subassembly. The optical subassembly containing a laser diode is sometimes called a transmitter optical subassembly and the optical subassembly containing a photodiode is sometimes called a receiver optical subassembly. In order to provide a pair of parallel, straight optical paths between a pair of optical fibers and a pair of optical subassemblies the distance between the optical fibers must be the same as the distance between the optical elements, which in turn is limited by the width of the optical subassemblies. For optoelectronic transceivers using SC duplex optical connectors, the fiber pitch at the interface to the transceiver is 12.7 mm, or more than ten times the fiber pitch presented by MT-RJ connectors.
Referring to FIGS. 1A and 1B, there is shown an embodiment of a prior art optical assembly of an optoelectronic transceiver using standard TO header based packages 114 (i.e., the TO packages 102 are embedded in port assemblies) and a pair of optical fibers 108, 112 that are coupled to the transceiver by an SC duplex connector (not shown). The laser diode 104 and the photo diode 110 are each housed in a standard TO package 102 having a window 106. The laser diode 104 transmits an optical signal to the transmitting fiber 108 and the photodiode 110 receives an optical signal from the receiving fiber 112. A focusing lens 116 is positioned in each of the two optical paths. In the configuration shown in FIGS. 1A and 1B, the transmitting fiber 108 and the receiving fiber 112 are about 12.7 mm apart at the connector interface. In other configurations, when the two TO packages 102 are spaced together as closely as possible, the transmitting fiber 108 and the receiving fiber 112 are no closer than 5 mm apart at the connector interface.
Due to the relatively large diameters of standard optical subassemblies, the pair of optical fibers at the interface of a transceiver in which the transmitter and receiver optical subassemblies are placed side by side must be at least 5 mm apart, and typically must be even further apart (e.g., at least 10 mm apart). Thus, it would seem that in order to couple the fibers in an MT-RJ connector to the optical subassemblies of an optoelectronic transceiver, a new optical element housing might be devised for the optical subassemblies to enable the laser and photodiode elements to be positioned the same distance from each other as the fiber pitch of the MT-RJ connector. Alternatively, a single optical subassembly containing both the optical source and detector elements separated by 0.75 mm might be used. However, these approaches to coupling the pair of fibers in an MT-RJ connector to a pair of optical subassemblies are less than optimal. The electromagnetic fields associated with the electrical connections of the transmitter and receiver elements will tend to interfere with each other when they are close to each other (e.g., within 1 mm of each other), resulting in electrical crosstalk between the transmitter and receiver signals. Also, changing the optical element housing might require the use of higher precision, and thus more expensive, housing components. Finally, devices packaged in TO packages are presently in widespread use and may therefore be considerably less expensive than customized packaging solutions.
In summary, the present invention is an optical assembly that includes a first lens positioned along a first axis and configured to approximately collimate light from a light source along the first axis, and a second lens positioned along the first axis and configured to focus light from the first lens onto an optical target. Also, there is a third lens positioned along a second axis and configured to approximately collimate light from a second light source along the second axis. A reflector positioned along the second axis is configured to reflect light from the third lens onto a fourth lens, which is positioned on a third axis angled from the second axis and configured to focus light from the reflector onto an optical detector. Furthermore, the optical assembly is a single molded optic.
Another aspect of the present invention is an optoelectronic transceiver having an optical subassembly and the aforementioned optical assembly. The optical subassembly includes a light source and an optical detector.
The present invention minimizes electrical cross talk between the transmitter and receiver signals flowing through an optoelectronic transmitter while allowing the optical fibers to be spaced very close together. More importantly, the present invention provides simple optical paths between the optical elements and optical fibers, with no reflections in the transmitter optical path and only a single reflection in the receiver optical path.